The immune response can loosely be divided into two components: the humoral immune response which involves antibody formation, and the cell-mediated immune response which involves the activation of macrophages, natural killer (NK) cells, antigen-specific cytotoxic T-lymphocytes, and the release of various cytokines in response to antigen. Typically, B lymphocytes (B cells) are characterized by their role in antibody production; whereas T lymphocytes (T cells) are characterized by their role in cell-mediated immunity. However, B cells possess additional immune functions, including the production of cytokines, and the ability to function as antigen presenting cells (APCs).
Once generated, immune responses need to be regulated to prevent the responding effector cells from causing harmful effects. Immunoregulation has traditionally been thought of as a function of T cells. Functionally distinct regulatory T cell subsets have been identified and clearly defined. For example, helper T cells that up-regulate the immune response include T helper type 1 (Th1) cells that regulate cell-mediated immune responses, and T helper type 2 (Th2) cells that regulate the humoral immune response. Suppressor T cells crucial for the maintenance of immunological tolerance, currently referred to as T regulatory cells, include IL-10-producing T regulatory 1 (Tr1) cells, and TGF-β1-producing T helper type 3 (Th3) cells. Recent studies of autoimmune conditions gave rise to the notion that B cells may also participate in immunoregulation. However, regulatory B cell subsets have not been clearly defined.
Multiple roles for B cells have been implicated in autoimmune diseases. B cells, a major immune cell population, can play a pathogenic role in acquired immune responses by producing autoantibodies that drive the development of autoimmune diseases. Certain therapies developed for treating autoimmunity are aimed at depleting pathogenic B cells. However, the tools currently available are not specific for the pathogenic B cells and deplete most B cells. For example, B cell depletion in humans using CD20 monoclonal antibody (mAb) can be effective in treating patients with various autoimmune disorders, such as rheumatoid arthritis and lupus (Edwards et al., 2001, Rheumatol. 40:205-11; Edwards et al., 2005, Rheumatol. 44:151-56; El Tal et al., 2006, J. Am. Acad. Dermatol. 55:449-59; Anolik et al., 2004, Arth. Rheum. 50:3580-90; Stasi et al., 2007, Blood 110:2924-30). CD20 is a B cell-specific marker that is first expressed on the cell surface during the pre-B to immature B cell transition, but is lost upon plasma cell differentiation (Tedder & Engel, 1994, Immunol. Today 15:450-54; Uchida et al., 2004, Int. Immunol. 16:119-29). A recent phase II trial using CD20 antibodies indicates clinical efficacy in multiple sclerosis (MS) patients (Hauser et al., 2008, N. Engl. J. Med. 358:676-88). However, the mechanisms underlying the effect of B cell depletion on disease activity remains unknown. On the flip side, B cell depletion may exacerbate disease. Indeed, B cell depletion was recently found to exacerbate ulcerative colitis in human clinical trials (Goetz et al., 2007, Inflamm Bowel Dis. 13:1365-8) and may contribute to the development of psoriasis (Dass et al., 2007, Arthritis Rheum. 56:2715-8).
Over a decade ago, Janeway and colleagues (Wolf et al., 1996, J. Exp. Med. 184: 2271-2278) described studies designed to assess the role of B cells in the course of autoimmune disease by inducing acute experimental autoimmune encephalomyelitis (EAE) in B cell-deficient mice. EAE is an autoimmune disease of the central nervous system (CNS) that models human multiple sclerosis. Results showed that elimination of B cells did not prevent induction of autoimmunity. Instead, the lack of B cells seemed to exacerbate disease outcome, in that the B cell deficient mice did not fully recover as compared to wild-type mice. Thus, while B cells supply the autoantibodies thought to be responsible for disease, these investigators concluded that B cells are not required for activation of disease, and instead, that their presence is required to enhance recovery. More recently, it was reported that B cell IL-10 production correlated with recovery from EAE, a Th1-mediated autoimmune disease (Fillatreau et al., 2002, Nature Immunol. 3: 944-950). IL-10 is an immunoregulatory cytokine produced by many cell populations. IL-10 has been shown to suppress cell-mediated immune and inflammatory responses.
Other recent studies in mouse models indicate that B cells and IL-10 play a protective role in T cell-mediated inflammation, e.g., in Th2-mediated inflammatory bowel disease (Mizoguchi et al., 2002, Immunity 16:216-219), and in contact hypersensitivity (CHS) responses—a cutaneous inflammatory immune reaction that is mediated by T cells in sensitized individuals following subsequent contact with the sensitizing antigen (Enk, 1997, Mol. Med. Today 3:423-8). In particular, mice with B cells deficient for CD19 expression (CD19−/−) have augmented CHS responses (Watanabe et al., 2007, Am. J. Pathol. 171:560-70). IL-10 must be involved in protection since neutralizing IL-10 through mAb treatment enhances CHS responses, while systemic IL-10 administration reduces CHS responses (Ferguson et al., 1994, J. Exp. Med. 179:1597-1604; Schwarz et al., 1994, J. Invest. Dermatol. 103:211-16).
On the basis of these and other studies, it has been proposed that, like their T cell counterparts, B cells can be divided into functionally distinct regulatory subsets capable of inhibiting inflammatory responses and inducing immune tolerance by mechanisms that include IL-10 and TGF-β production, secondary antigen presentation, and interaction with other immune cells either directly or through secreted antibodies. (For reviews on the subject, see Mauri & Ehrenstein, 2007, TRENDS in Immunol. 29: 34-40; and Mizoguchi & Bhan, 2006, J. Immunol. 176:705-710).
However, it remains unclear whether regulatory B cells represent a unique regulatory lineage tasked with maintaining self-tolerance the way that naturally occurring regulatory T cells are. The generation of regulatory B cells has been reported in multiple mouse models of chronic inflammation, although their existence in normal mice remains unknown (Mizoguchi & Bhan, 2006, J. Immunol. 176:705-10). Despite the identification of a regulatory B cell subset generated in these mouse models, no definitive murine phenotype has been established and, in fact, only a general list of cell-surface markers associated with regulatory B cells exists (Mauri & Ehrenstein, 2007, Trends Immun. 29:34-40). Furthermore, the existence of regulatory B cells in humans remains a matter of speculation, and the potential phenotypic markers for human regulatory B cells are unknown (Mauri & Ehrenstein, 2007, Trends Immun. 29:34-40). A role for CD40 in the generation of regulatory B cells and the induction of IL-10 production by these cells has been postulated (Inoue et al., 2006 Cancer Res. 66:7741-7747). Nonetheless, it has yet to be proven whether CD40 can be directly targeted, i.e., with CD40 antibodies, as a means to generate regulatory B cells in vivo (Mauri & Ehrenstein, 2007, Trends Immun. 29:34-40).
Further complicating these issues, during immune responses, IL-10 is also secreted by multiple cell types, including T cells, monocytes, macrophages, mast cells, eosinophils, and keratinocytes, and can suppress both Th1 and Th2 polarization and inhibit antigen presentation and proinflammatory cytokine production by monocytes and macrophages (Asadullah et al., 2003, Pharmacol. Rev. 55:241-69). Clearly, it is unknown whether multiple B cell populations or a novel B cell subset regulates inflammatory responses, whether regulatory B cells produce IL-10 or other cytokines directly, whether regulatory B cells have potent activities in vivo, whether humans possess regulatory B cells, how regulatory B cells can be activated and/or expanded, and the role of regulatory B cells in disease. To advance therapeutic application, subsets of immunoregulatory B cells need to be better defined and their phenotype will need to be more closely correlated with their function in vivo.